Did U.S. make mistake in skipping vaccine additive?

As U.S. health officials struggle to vaccinate tens of millions of Americans against the pandemic of swine flu, some are looking regretfully at one easy way to instantly double or triple the number of doses available -- by using an immune booster called an adjuvant.

These additives, often as simple as an oil and water mixture, broaden the body's response to a vaccine, reducing the amount of active ingredient called antigen needed. They are widely used in European flu vaccines as well as in Canada. But not in the United States -- even though the federal government has spent nearly $700 million buying them.